1. Field of the Invention
This invention relates to an apparatus for recording information which has a function of encrypting the information. In addition, this invention relates to a method of recording information which has a step of encrypting the information. Furthermore, this invention relates to an apparatus for recording and reproducing information which has a function of encrypting the information and a function of decrypting the encryption-resultant information. Also, this invention relates to a method of recording and reproducing information which has a step of encrypting the information and a step of decrypting the encryption-resultant information. In addition, this invention relates to a recording medium.
2. Description of the Related Art
In a common-key cryptosystem (a secret-key cryptosystem), a key for encrypting original data and a key for decrypting the encryption-resultant data are the same. Thus, a common key is used for encrypting original data and also decrypting the encryption-resultant data. Generally, the common key is secret.
It is known that original audio-visual data representing one movie content are encrypted in response to only one common key. In the event that the common key is disclosed, the original data can be reproduced from the encryption-resultant data by use of the disclosed key so that the whole of the movie content can be illegally played back.
A first conceivable system implements encryption on a block-by-block basis. Specifically, original audio-visual data are divided into successive equal-size blocks. The data blocks are encrypted in response to different keys, respectively. A legitimate encryption side and a legitimate decryption side hold signals representing the keys. In the event that one of the keys is disclosed, only a portion (a block) of the original audio-visual data which has been encrypted by use of the disclosed key can be illegally recovered.
A second conceivable system improves over the first conceivable system. In the second conceivable system, signals representative of keys for encrypting data blocks are also encrypted. The encryption-resultant key-representing signals are stored.
A third conceivable system improves over the first conceivable system. The third conceivable system uses an initial key in encrypting a first data block. Second and later keys are generated from the initial key according to prescribed rules. Second and later data blocks are encrypted in response to the second and later keys. It is unnecessary for a legitimate encryption side and a legitimate decryption side to hold signals representing the second and later keys.
A fourth conceivable system divides original audio-visual data into successive equal-size blocks. The fourth conceivable system uses a seed of keys. For every audio-visual content, a signal representing the seed is encrypted by a public-key cryptosystem. Keys are calculated from the seed-representing signal according to a secret function. The data blocks are encrypted in response to the calculated keys, respectively. The calculated keys are separated into a group of odd keys and a group of even keys. The odd keys and the even keys are alternately used. A key change timing is controlled by 1-bit information indicating which of an odd key and an even key should be used.
A fifth conceivable system is modified from the fourth conceivable system. The fifth conceivable system uses an initial value protected by a public-key cryptosystem. Firstly, a parameter value is set to the initial value. Thereafter, the parameter value is counted up or incremented each time change between an odd key and an even key occurs. The parameter value is processed by a one-way function. A portion of the processing-resultant value is used as a key for encrypting a data block.
It is assumed that encryption-resultant data blocks generated by one of the third, fourth, and fifth conceivable systems are recorded on a recording medium. During high-speed playback, some of the encryption-resultant data blocks are reproduced from the recording medium while others are skipped. In this case, to determine keys for the reproduced encryption-resultant data blocks, it is necessary to detect the conditions of skipping data blocks in the recording medium. Therefore, the determination of the keys is troublesome.